I have a 1988 D100 4X4 318. My front wheels will not engage. The front drive shaft turns but will not engage at the axle. I pulled the vacum lines of the actuator on the axle and found that only one line has vacum when in 2 wheel drive. Any one have any sugestons?
Todd

 

Assuming the axle is OK mechanically I'd highly recommend a cable conversion kit. The CAD setup can cause trouble at the most inopportune times. I swapped an early 80's axle under mine did away with the whole aggravating mess.

Anyhow if I remember right there's 2 nipples on the diaphram. Correct me if I'm wrong it's been a while.
Swap the hose to the other nipple and see if it engages. Probably got a screwed up vacuum switch assm or a split hose someplace. Hate 'em.

 

The switch in the transfer case is supposed to switch over the vacuum from one line to the other when you shift into 4 wheel drive. The right line gets vacuum when the transfer case is put in 4 wheel drive (line toward passenger side). When you switch into 2 wheel drive the vacuum is supposed to go to the other line. The previous line then goes to a relief line so the actuator can work correctly. You may have a bad transfer switch. You may have a hole in the vacuum line. You can hook a line direct like I did from the manifold to the vacuum canister and then to the right side of the actuator but you will always have the axles locked until you manually swap the hose to the other side of the actuator. If you do this use the same manifold line that the transfer case used. There is one line to the transfer case with vacuum all the time. There is also a breather line. They both go down the passenger side next to the firewall. Make sure the exhaust didn't burn a hole in the vacuum lines.The unused side of the actuator must be open to reduce pressure on the back side.